Storage pools should be explicitly exported to indicate that they are ready to be migrated. This operation flushes any unwritten data to disk, writes data to the disk indicating that the export was done, and removes all knowledge of the pool from the system.
If you do not explicitly export the pool, but instead remove the disks manually, you can still import the resulting pool on another system. However, you might lose the last few seconds of data transactions, and the pool will appear faulted on the original machine because the devices are no longer present. By default, the destination machine refuses to import a pool that has not been explicitly exported. This condition is necessary to prevent accidentally importing an active pool that consists of network attached storage that is still in use on another system.